1. Field of the Invention
The present invention relates to a fuel-reforming apparatus, for producing a reformed gas containing hydrogen by reforming reformable fuel containing hydrocarbon.
2. Description of the Related Art
A fuel cell stack has been developed, which comprises, for example, a plurality of stacked fuel cells interposed by separators, the fuel cell including an anode electrode and a cathode electrode provided opposingly with a solid polymer electrolyte membrane interposed therebetween. Such a fuel cell stack has been practically used for a variety of applications.
The fuel cell stack of this type is constructed as follows. That is, a reformed gas (fuel gas) containing hydrogen, which is produced by reforming hydrocarbon, for example, an aqueous methanol solution with steam, is supplied to the anode electrode, while an oxygen-containing gas (air) is supplied to the cathode electrode. Thus, the hydrogen gas is ionized, and it flows through the solid polymer electrolyte membrane. Accordingly, the electric energy is obtained at the outside of the fuel cell.
The steam reforming reaction, in which the aqueous methanol solution is reformed to produce the reformed gas containing hydrogen as described above, is an endothermic reaction represented by CH3OH+H2Oxe2x86x92CO2+3H2. Therefore, a complicated heat transfer structure is usually incorporated in the reformer in order to supply an amount of heat necessary for the reforming reaction. As a result, the structure is complicated.
In view of the above, an endothermic reaction apparatus is known, for example, as disclosed in Japanese Laid-Open Patent Publication No. 3-122001, comprising an endothermic reaction unit including a cylindrical container for surrounding a combustion chamber having a burner disposed at one end and a reaction chamber containing a catalyst for facilitating the endothermic reaction charged along an inner cylinder of the cylindrical container, and a heat-insulating container for accommodating a plurality of endothermic reaction units as described above. In this apparatus, the reaction chamber is formed along an inner wall of an intermediate cylinder provided in the cylindrical container. The intermediate cylinder and a brim-shaped partition plate are used to form a preheating chamber for raw material gas communicating with the inlet side of the reaction chamber and a reproducing chamber communicating with the outlet side of the reaction chamber. Further, a cover is provided to form a combustion gas passage on the outer circumferential side of the preheating chamber. In this apparatus, the thermal energy of the combustion gas is effectively utilized to preheat the raw material gas so that the heat consumption amount of the combustion chamber is reduced.
However, the apparatus concerning the conventional technique described above comprises the cylindrical container provided with the inner cylinder and the outer cylinder, the intermediate cylinder arranged in the cylindrical container, and the brim-shaped partition plate provided for the intermediate cylinder. Therefore, a problem is pointed out in that the number of parts is considerably increased, and the system is complicated.
Further, the outer circumferential wall is constructed by the heat-insulating container which has a relatively large wall thickness. Therefore, a problem arises in that the whole apparatus becomes large.
In another viewpoint, for example, as disclosed in Japanese Laid-Open Patent Publication Nos. 9-315801 and 7-335238, a method is known, in which oxygen is supplied to a raw material fuel gas containing hydrocarbon to perform the oxidation reaction as the exothermic reaction, and the amount of heat released by the oxidation reaction is utilized so that the reforming reaction as the endothermic reaction is performed for the raw material fuel gas. Accordingly, an advantage is obtained in that the structure can be simplified.
In general, the velocity of the oxidation reaction is larger than the velocity of the reforming reaction. Therefore, the temperature on the inlet side of the reforming catalyst tends to increase, while the temperature on the outlet side of the reforming catalyst, which is important for the reforming reaction, tends to decrease. However, in the conventional technique described above, the reforming catalyst (composed of pellets) is formed to be lengthy in the flow direction of the gas. For this reason, the difference in temperature is large in the flow direction of the gas in the reforming catalyst. Therefore, a problem is pointed out in that it is impossible to realize the desired reforming reaction over the entire region of the catalyst layer. Further, the pellet is inconvenient in that the compact property is inferior, and it is extremely difficult to obtain an equivalent temperature over the reforming catalyst.
In the case of such a system, when it is intended to control the temperature on the gas outlet side of the reforming catalyst in order to efficiently perform the reforming reaction, it is feared that the temperature on the gas inlet side of the reforming catalyst may be locally increased to be not less than the heat resistant temperature of the reforming catalyst. For this reason, a problem is pointed out in that the concentration of produced carbon monoxide is increased, and the reforming catalyst is quickly subjected to thermal deterioration. On the other hand, when it is intended to set the temperature on the gas outlet side of the reforming catalyst in order to avoid the thermal deterioration of the reforming catalyst, an inconvenience arises in that the reaction efficiency of the reforming catalyst is extremely lowered.
A structure is usually adopted for the reforming catalyst, in which plate-shaped reforming catalyst layers and catalytic combustion chambers are alternately stacked (see, for example, Japanese Laid-Open Patent Publication No. 8-253301). However, such a reforming catalyst layer is generally designed to have a rectangular plate-shaped configuration. Therefore, the entire case for constructing the reformer is rectangular. For this reason, the following problem arises. That is, the stress tends to concentrate in the case, the case inevitably has a large wall thickness, and it is impossible to miniaturize the entire reformer.
On the other hand, when the steam reforming for the aqueous methanol solution is started, it is necessary to heat the reforming catalyst to a predetermined temperature. For this purpose, an apparatus, which is disposed at the outside of the reformer, is usually used to supply the heat such as steam to the reformer. However, a compact reformer especially having a high efficiency is required for the fuel cell stack to be carried on vehicles or automobiles. In such a case, it is impossible to adopt the structure as described above.
As shown in FIG. 34, the reformer 1 for reforming the aqueous methanol solution is sometimes designed such that the cross-sectional area of a flow passage 2 for methanol mixed with steam (hereinafter referred to as xe2x80x9creformable fuel gasxe2x80x9d) is smaller than the cross-sectional area of the reforming catalyst section 4. In this arrangement, in order to uniformly supply the reformable fuel gas to the entire surface of the reforming catalyst section 4, there is usually provided a region for widening the cross-sectional area of the flow passage, i.e., the cone section 6 on the upstream side of the reforming catalyst section 4.
However, if the cone section 6 is not designed to be sufficiently long in the flow direction of the reformable fuel gas, the reformable fuel gas is not delivered uniformly over the entire cross-sectional area of the reforming catalyst section 4. As a result, the reformable fuel gas flows through only a part of the reforming catalyst section 4. It is feared that the whole surface of the reforming catalyst section 4 cannot be utilized effectively. Therefore, in fact, it is necessary to use a sufficiently long cone section 6. A problem is pointed out in that the reformer 1 has a considerably large size.
On the other hand, the introducing hole for supplying the reformable fuel gas to the flow passage 2 of the reformer 1 is usually provided at one place. However, considering the property of the fuel cell stack that it is carried on the vehicle, it is desirable that the reformer 1 is of the transverse or horizontal type as shown in FIG. 34 so that the reformable fuel gas is allowed to flow in the lateral or horizontal direction. On the contrary, if the introducing hole is provided at one place, a problem arises in that it is extremely difficult to uniformly supply the reformable fuel gas to the whole of the reforming catalyst section 4, due to the influence of the self-weight of the reformable fuel gas.
In the case of the fuel-reforming apparatus for producing the reformed gas by reforming the aqueous methanol solution, carbon monoxide (CO) and unreacted hydrocarbon components exist in a mixed manner in the reformed gas components produced during the warming-up process after the start-up. If the reformed gas mixed with CO is supplied to the fuel cell stack, the CO poisoning of the catalyst occurs on the anode electrode.
In order to dissolve the inconvenience described above, a fuel cell system is known as disclosed, for example, in Japanese Laid-Open Patent Publication No. 8-293312. In this conventional technique, when the fuel cell is in the start-up operation, if any one of the detected temperature and the detected CO concentration which are obtained by a temperature sensor and a CO sensor respectively is deviated from an allowable temperature range or an allowable CO concentration which is prescribed on condition that the fuel cell is in the steady state, then a flow passage-switching valve is used to switch the supply destination of the hydrogen-rich gas supplied from the reforming unit, from the fuel cell to a burner so that the hydrogen-rich gas containing high concentration CO is not supplied to the fuel cell.
However, in the conventional technique described above, methanol and water are supplied into the reforming unit, and the reformed gas containing hydrogen gas is produced by reforming methanol with steam. The steam reforming reaction is an endothermic reaction. The reforming unit is provided with a burner for heating the reforming unit to a temperature appropriate to perform the reforming reaction of methanol. However, the following problem is pointed. That is, a considerably long period of time is required for the warming-up operation to heat the reforming unit up to the temperature appropriate to perform the reforming reaction, because the reforming unit is heated to the predetermined temperature (for example, about 250xc2x0 C. to 300xc2x0 C.) by using the burner.
A general object of the present invention is to provide a fuel-reforming apparatus composed of a simple system which makes it possible to smoothly perform a desired reforming reaction and easily miniaturize the entire apparatus.
A principal object of the present invention is to provide a compact fuel-reforming apparatus having a good thermal efficiency in which the start-up operation is smoothly performed.
Another principal object of the present invention is to provide a fuel-reforming apparatus and a method for controlling the same which make it possible to reliably avoid the occurrence of thermal deterioration of a reforming catalyst section and smoothly perform a desired reforming reaction.
Still another principal object of the present invention is to provide a fuel-reforming apparatus composed of a simple system which has an effective heat-insulating function and which can be produced economically.
Still another principal object of the present invention is to provide a fuel-reforming apparatus and a method for controlling the same which make it possible to greatly shorten the warming-up operation time by using a simple system.
Still another principal object of the present invention is to provide a fuel-reforming apparatus and a method for controlling the same which make it possible to perform the warming-up operation efficiently and economically.
Still another principal object of the present invention is to provide a fuel-reforming apparatus and a method for controlling the same which make it possible to reliably detect the reaction state in a reforming catalyst section by using a simple and inexpensive system.
The above and other objects, features, and advantages of the present invention will become more apparent from the following description when taken in conjunction with the accompanying drawings in which a preferred embodiment of the present invention is shown by way of illustrative example.